Why won't my mask display a full image?
The reason it is not working like you expected is because you are using a mask. Right now all you are getting is a black and white mask that has no transformational information so all it is doing is "hiding" the part of the image outside of your mask.

What you want to do is use a Plane track and a Plane Track Deform Node. After setting up the plane track, that node will deform whatever image you give it as an input to match the skew of the plane track. 


Switch to the Tracking mode in the Movie Clip Editor, add 4 tracking markers.
With the 4 markers selected go to the Solve tab in the Tool Shelf and click the Create Plane Track button.
Drag the corner points of the newly created plane track to match your frame.
Now in the compositor set up your nodes to look something like this:

The Plane Track Deform Node is the heart of this, that is where all the data from the actual plane track gets in to the compositor to distort the image.




My answer here show the whole process of setting up the track and the needed nodes in more detail.